Precise Handling of Salts and Tackling the Challenges of Triassic and Zechstein Formations: Insights from the Elephant 2.0 Database

The work of the Elephant project was carried out by Rune ?ver?s , Vita Kalashnikova , Barbara Eva Klein , and Marcin Kaluza , with special recognition for the latest interpretation work Alena Finogenova and Anna Ivanova (Gladiy) . Petrophysical support was provided by Carl Fredrik Gyllenhammar, PhD and Tatyana Nekrasova with broad-ranging support, including data from Lime Petroleum AS , coordinated by Vladimir Sopivnik .

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About the Elephant Database

The Elephant Database is a unique, massive resource created to support exploration in the North Sea. The merge is suitable for inversion, incorporating:

• 200 true amplitude surveys, denoised and deghosted with phase control from wells (at least 3 per survey), plus 2D regional lines, preserving frequency.
• 815 manually tied wells, enabling interpretation of 10 key surfaces—sufficient to build initial velocity and density models.
• Proprietary Rune Inversion, a full-time seismic inversion technique.

After two years on the market, the database has already generated sufficient statistics to demonstrate its efficiency. Our first focus was on the main hydrocarbon (HC) window, where we successfully computed sand volume and porosity as primary working parameters.

Our Methods: Breaking Conventions

Our approach is unconventional:

• Working with post-stack data, we derived non-linked Vp and Density using AI algorithm and constraints.
• By identifying relationships between velocity (Vp), density, and clay volume (Vclay) for each well, we computed lithology cubes.

Well Correlation

Thanks to Cark Fredric Gyllenhammar’s detailed well database Carl Fredrik Gyllenhammar, PhD , which covers the Norwegian Shelf, we gained precise clay curve computations. His database has unique analysis for salinity, clay, missed pays and hydrocarbons. His approach for Vclay computation from Vp and Density served as the basis for creating a Vclay equation adjusted for each stratigraphic unit. Furthermore, we computed Porosity and Best Quality Sand cubes

Here is an example of Gyllenhammar's Vclay curve and the use of his equation, where we perfectly reconstruct Vclay from Vp and density (see tracks 2-5).

Ambitions for Elephant 2.0

With Elephant 2.0, we aim to take the project even further by:

• Expanding the HC window to compute formation-based sand volumes that match well data more accurately.
• Improving surface interpretations and implementing high-density horizon picking for better structural visualization.

Our work includes detailed structural grids for key horizons, such as:

• Base Triassic (merging Zechstein Formation with the basement)
• Top Triassic
• Bathonian Top (Brent/Fladen Group)
• Base Cretaceous Unconformity
• Top Cromer Knoll Group
• Top Agat Formation

These efforts focus on pinch-outs, erosion zones, and stratigraphic traps, improving reservoir modeling and exploration outcomes.

Salt Structures: A Central Focus

Salt formations were one of a major focus of our work. The interpreted surface merges the top of the Permian Zechstein Formation with the Basement, representing the base of the interval of interest spanning the Triassic, Jurassic, and Cretaceous ages. Depending on the location within the Zechstein Permian Basin, the lithology of the formation varies significantly—carbonates and sandstones dominate intra-basin highs, while evaporites such as halite and anhydrites prevail in the slopes and depressions of the paleo-Permian basin.

Mobilized halite forms large diapirs, while anhydrite layers complicate seismic imaging within the Zechstein Supergroup. Bright reflectors within halite intervals often indicate lithological changes. Due to gravitational gliding and differential loading, mobile salts form a variety of structural features.

Clark and Jackson (1998-2022) have recently published work exploring how syn-depositional salt flow influenced the post-depositional deformation of both the salt and its overburden. Our observations align with these findings, as we see numerous Triassic minibasin structures with strata onlapping the edges of salt deformed by differential loading. By analyzing the relationships between reflectors within these basins, we identified areas where bright reflectors correspond to Triassic sands, rather than salt. These sands have hosted oil and gas discoveries in the Southern Viking Graben, confirmed by well data.

Salt-Related Structures in the Central North Sea

In the Central North Sea, we observe grabens formed by the collapse of salt diapirs. Often, Jurassic and Cretaceous strata onlap the sides of post-depositional salt diapirs.

Salt structures began to form during syn-depositional Permian rifting. Early Permian rifting, associated with the development of the Central Graben, played a key role in shaping the location and extent of the Zechstein Supergroup evaporites. This rifting directly influenced the lithological distribution within the Zechstein Supergroup:

• Carbonate- and anhydrite-rich units were deposited along basin margins and intra-basin structural highs during highstands.
• Halite- and K–Mg-rich salt units accumulated in deeper basins during lowstands (Joffe and Jackson, 2022).

In structural highs such as Utsira, thin Jurassic and Triassic strata are patchily distributed. On the apex of the highs, Cretaceous sediments often directly overlie the basement, as Jurassic and Triassic strata are fully eroded.

Confirming Observations with Data

We used well data to confirm these patterns, particularly near the Utsira High, where numerous patchy grabens are preserved. Published gravity data (Jan Erik Lie and Espen Harris Nilsen, 2016) indicates the location of grabens within the granitic basement, helping us differentiate them in areas with unclear seismic imaging.

These grabens are primarily filled with Permian and Triassic sediments. Our focus was on grabens containing Triassic-age sediments, as they are associated with significant oil and gas discoveries, such as Edvard Grieg and Luno.

Our careful, aforementioned work has allowed us to make interesting observations, such as the one presented here.

On the western slope of the Utsira High, sand intervals can be delineated that likely correspond to the Triassic Hegre Group and the Jurassic Sleipner Formation. The best reservoir attributes indicate predicted porous sands similar to the Dagny discovery and those observed in wells 15/6-10.

The distribution of these sands is influenced by syn-depositional rifting and salt tectonics. This evolving structural template controls the development of accommodation space, topography, and sediment routing, which collectively influence the facies distributions of sands within the Southern Viking Graben.

Key References

Our work builds on the following influential studies:

• Clark, J.A., Stewart, S.A., & Cartwright, J.A. (1998): Evolution of the NW margin of the North Permian Basin.
• Marín, D., Cardozo, N., & Escalona, A. (2022): Zechstein Group compositional variation for underground storage.
• Jackson, C., Evrard, E., Elliott, G., & Gawthorpe, R. (2014): Lithology distribution in the Zechstein Supergroup.
• Joffe, A., Jackson, C.A.-L., & Pichel, L.M. (2022): Syn-depositional halokinesis and minibasin genesis.

Join the Journey

The Elephant Database is revolutionizing seismic and well data interpretation for the North Sea. With Elephant 2.0, our innovative methods and focus on salts, Triassic, and Zechstein systems are empowering geologists to uncover new exploration opportunities.

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